1. Field of the Invention
The present invention relates to hydrostatic drive systems for vehicles.
2. Description of the Prior Art
Vehicles with hydrostatic transmissions are in widespread use. Drive trains of this type typically include a reversible variable displacement hydraulic pump which is driven by an engine. In one conventional design, fluid from the hydraulic pump is provided to a single hydraulic motor which is mounted to a gear box. The gear box is coupled to each of two sets of drive wheels through universal joint shafts, differentials, and axles. Vehicle speed and direction are controlled by the volume and direction of fluid displaced by the pump. A separate frame to support the vehicle and hold the drive train is required. Rotating drive train parts are exposed to damage by obstacles and wrapping by debris.
The hydraulic drive control system of a skid steer loader is disclosed in the Bauer U.S. Pat. No. 4,074,782. This drive system includes a transmission case with a hydraulic motor mounted to each of its opposite sides. Each hydraulic motor is coupled to both the forward and rearward wheels on that side of the loader by a gear reduction mechanism and chain linkage. The gear reduction mechanism and chain linkage are enclosed within the transmission case. Each motor is coupled to and separately powered by its own hydraulic pump.
The Jeffery U.S. Pat. No. 3,484,964 discloses a hydrostatic transmission system in which a front motor and rear motor are connected in parallel with a main hydraulic pump. When one axle on a vehicle of this type loses traction, all of the hydraulic fluid will flow to the motor from which it is driven, taking the path of least resistance. No pressure or torque remains for the axle with traction. The machine can stall under these circumstances.
The Chichester et al. U.S. Pat. No. 3,910,369 discloses a hydrostatic transmission system for an articulated vehicle. Like that of the Jeffery patent discussed immediately above, this system utilizes two hydraulic motors which are connected in parallel to the hydraulic pump. Flow dividers are provided in the Chichester et al. system to maintain equal fluid flow to each motor. Should one axle or wheel lose traction, the pump will continue to supply fluid at high pressure to the axle or wheels with traction. However, the energy in the fluid provided to the axle or wheels with no traction is lost as heat since no work is done. Energy is therefore wasted.
It is evident that there is a continuing need for improved hydrostatic transmission systems for vehicles. In particular, there is always room for more compact and energy-efficient transmission systems.